About this tool
Gestational age is counted from the first day of the last menstrual period (LMP), not from conception - which is why a pregnancy lasts about 40 weeks (280 days). Type the LMP and, if your cycle is not 28 days, its length; or use the date and the measurement of an ultrasound, or the conception date. The calculator shows the weeks and days, the trimester, the estimated due date (Naegele's rule: LMP + 280 days) and the dates of the usual prenatal milestones, such as the first-trimester scan, the anatomy scan and the glucose test. It is an estimate for information only: the dating set by your doctor prevails. Nothing you type leaves your browser.
How to use it
- 1 Choose the basis: last period, ultrasound or conception date.
- 2 Type the date (and the cycle length or the weeks and days measured on the ultrasound).
- 3 Read the gestational age, the trimester and the due date.
- 4 Check the prenatal milestones: what is done, what is now and what is next.
Frequently asked questions
How is the due date calculated?
By Naegele's rule: first day of the last period + 280 days (40 weeks). With a cycle longer or shorter than 28 days, the date moves by the difference - a 32-day cycle adds 4 days.
Why does the pregnancy count from the last period?
Because the LMP is a date most people know, while the exact day of conception is rarely known. Conception usually happens about two weeks after the LMP, so at conception the gestational age is already about 2 weeks.
Ultrasound or last period: which is more accurate?
A first-trimester ultrasound (up to about 13 weeks) is the most accurate way to date a pregnancy. When it differs a lot from the LMP, doctors usually follow the ultrasound - use the ultrasound option with the date and the weeks and days it measured.
Will the baby be born on the due date?
Rarely on the exact day: most babies are born between 37 and 42 weeks. The due date is a reference for following the pregnancy, not an appointment.
Guide
Gestational Age: How to Count Pregnancy Weeks and Work Out the Due Date
Why pregnancy is counted from the last period, Naegele's rule for the due date, when an ultrasound changes the dates and the main prenatal milestones.
Read the guide →Embed this tool
Add this free tool to your own website — copy the code below. It stays up to date automatically.
Tools for your site →More in Health
BMI Calculator
Calculate Body Mass Index from height and weight, with the WHO category and a healthy-weight range.
Calorie CalculatorNew
Work out your basal metabolic rate and daily calories for your activity level, with targets to lose, keep or gain weight and how much water to drink.
Menstrual Cycle CalculatorNew
Find out when your next period is due, your fertile days and the estimated ovulation day, with the next six cycles on a calendar.
Pregnancy Weight Gain CalculatorNew
See the recommended weight gain for your pre-pregnancy BMI, the range for your current week and where your gain sits, on a week-by-week chart.
Percentage Calculator
Percent of a number, percent change, discount price, and tip splitting - five common questions in one tool.
Compound Interest Calculator
See how savings grow over time with regular contributions and compound interest.